The present invention relates generally to methods and apparatus in paper machines and, more particularly, to methods and apparatus in the head box of a paper machine for controlling the distortion of fibre orientation in the paper web.
It is known that the speed of the discharge flow of the pulp suspension from the head box must be uniform in the transverse direction of the paper machine. It is also known that if the transverse speed of the pulp suspension flow discharging from the head box is unduly high, the quality of the paper produced may be detrimentally affected. In particular, an unduly high transverse speed in the discharge flow of the pulp suspension results in increased lateral wave formation at the lateral portions of the web. However, paper production is normally subject to requirements that the paper produced be homogeneous over the entire width of the web with respect to grammage, formation and strength properties so that as little as possible of the web edges must be cut off.
In order to meet these requirements, it has been suggested to remove a small portion of the pulp suspension flow through both of the side walls of the discharge channel of the head box before the suspension flow is discharged onto the forming wire. See, for example, Finnish Pat. No. 43.812 (U.S. Pat. No. 3,434,923) of Beloit Corporation. Another contrary solution has been suggested wherein an additional flow of water is passed through the side wall of the head box and in this connection reference is made to Finnish Pat. No. 30,095 (U.S. Pat. No. 2,956,623) of Valmet Oy.
The above-described requirements imposed on paper production have been increased and new requirements for the uniformity in the structure of fine paper have resulted from the recent development of certain printing methods, such as sheet-heating copying developed by Xerox and continuous-formedheating copying. These increased requirements are essentially due to the rapid and intensive heating of the sheet that takes place during the printing process. These new printing methods impose the particular requirement that the main axes of the directional distribution or orientation of the fibre network in the paper should coincide with the directions of the main axes of the paper and that the orientation should be symmetrical with respect to these axes.
Sufficient satisfaction of the particular requirement described above over the entire width of the web has not been possible in practice by means of the above-described prior art suggestions nor by means of any other known construction of the paper machine head box. For example, areas are usually present in the web which are not acceptable in view of the requirement described above. Paper produced by conventional methods are generally subject to the deficiencies of diagonal bending of the sheets or "falling" of a stack of forms.
Studies conducted by applicants' assignee have shown that it is possible to obtain the required symmetry of fibre orientation by ensuring that the transverse speed of the pulp suspension being discharged from the head box does not exceed about 2 to 3 cm/s. Since the transverse flow of pulp suspension is produced in the discharge channel of the head box as the uneven main flow profile is attenuated, the majority of effort must be directed to obtaining uniformity of the speed profile in the direction of pulp suspension flow after the turbulence generator. Even if it were possible to construct the distribution system of the head box in the correct manner and to construct turbulence generators so precisely that the transverse speed requirements are met, such constructions would be so costly in manufacture as to be commercially unprofitable.